Power-assisted steering having a gear mechanism

ABSTRACT

A power-assisted steering system includes a gear mechanism, coupled to a steering column, with a gear and a mating gear. The two transmission gears have teeth ( 4, 5 ) via which they engage with each other, movement and power being transmitted from one gear to the other gear via effective profiles of their tooth faces ( 11, 12 ). In order to provide a gear mechanism that has an enhanced load-bearing capacity and at the same time can be manufactured in a simple and economical way, the effective profiles of the tooth faces ( 11, 12 ) of the gear and the mating gear contact over the height (h 4 , h 5 ) of the teeth ( 4, 5 ).

BACKGROUND OF THE INVENTION

[0001] The present invention relates to the field of steeringmechanisms, and in particular to a power-assisted steering system havinga gear mechanism with a gear and a mating gear.

[0002] Power-assisted steerings has come into regular use in motorvehicles to assist with manual steering movements that a driver performson the steering wheel. Power-assisted steering systems includehydraulics to minimize the steering forces and reduce steering,especially at low speeds or when the vehicle is stationary.Power-assisted steering improves driver comfort, especially in parkingand maneuvering, and in city traffic. In addition, steering systems arebeing developed that are electrical rather than hydraulic systems.

[0003]FIG. 1 is a pictorial illustration of a power-assisted steeringsystem 100 with electromechanical steering assistance. FIG. 1 depicts asteering wheel 102 connected via a steering column 104 to a gearmechanism that has a worm gear 106 and a worm 108. In addition, aworm-drive electric motor 110 is coupled to the gear mechanism. Thisgear mechanism is connected via a drag link 112 to a steering rack 114of the motor vehicle. A tie rod 116 is coaxial with the steering rack114.

[0004] Gear mechanisms generally transmit a rotary movement of one shaftto another, which frequently occurs with conversion of a torque. Throughteeth meshing with one another, positive connection between the shaftsis provided and gear mechanisms ensure compulsory, non-slip transmissionof the rotary movement, or torque.

[0005] Gears with involute tooth design are in almost exclusive use inmechanical engineering. In an involute tooth design, the effectiveprofiles of the tooth faces (i.e., the tooth face profiles that comeinto contact with one another when the teeth mesh and through whichforce is transmitted) are involutes of a circle. That is, they describea curve obtained by constructing a tangent at points of a circle anddeducting on the tangents the length of the arc from the point ofcontact of the tangent with the circle up to a certain fixed point ofthe circle. In the case of externally toothed gears, the effectiveprofiles of an involute tooth design are convex.

[0006] Gears with involute tooth design can be made in a relativelysimple and precise manner. Advantage of this tooth geometry is thatvarious tooth shapes and axis spacings can be made with the same tool byshifting the profile. In operation, gears with involute tooth design aredistinguished by the fact that the direction and the magnitude of thetooth normal force is constant during the engagement of the teeth,resulting in uniform loading of the entire mechanism, in particular ofthe bearings of the mechanism.

[0007] Gear mechanisms have a variety of uses. They are used both inprecision technology and in vehicle construction, for example insteering assistance systems.

[0008]FIG. 2 depicts a worm drive 200 with a spur-gear-shaped worm gear200 and a worm 204 engaging with worm gear 202, each with involute toothdesign. In operation (i.e., when the teeth of worm 204 and worm gear 202mesh with one another) the teeth of worm gear 202 make contact with theteeth of worm 204 at a point 206. This contact leads to a high loadingof the teeth at this point, which, depending on the material pairing,can lead to severe wear and, in the extreme case overloading of theteeth. The load-bearing capacity of gears with a point contact is thuslimited.

[0009]FIG. 3 depicts a worm gear mechanism 300 having a worm gear 302and a worm 304 that engages the worm gear 302. In contrast to the wormgear mechanism depicted in FIG. 2, the worm gear 302 is globoidal ratherthan cylindrical in shape. As a result, the worm gear 302 contact isalong a line 306 over the width of the teeth, so the load transmitted bythe teeth is distributed over a larger area. This reduces the loadingper unit area of the individual teeth, so the load-bearing capacity ofthe gears is increased. As a consequence, both wear and the danger ofoverloading of the teeth are decreased.

[0010] Machining is required to make a globoidal worm gear sincegloboidal worm gears have undercut regions. However, machining increasesthe cost of manufacturing in comparison to other techniques for makinggears, such as for example injection molding. In addition, the assemblyof worm gear mechanisms with globoidal worm gears is more expensivebecause the worm gear can only be mounted in the radial direction, andnot in the axial direction. Radial insertion of the worm gear requiresmore space than axial insertion, and may damage the worm gear if theworm gear is not moved toward the worm in the correct angular position.This is particularly true when the worm gear is made of a material withlower strength than the material of the worm. In addition, the worm andthe worm gear must be positioned precisely relative to each other so theteeth mesh properly. Another drawback is that if the angle between theaxes of the worm and the worm gear is not equal to 90°, the worm gearmust be made less globoidal. As a result, the linear contact area 306becomes smaller, which in turn has a negative effect on the load-bearingcapacity.

[0011] German Patent DE4107659A1 discloses a worm gear mechanism inwhich the worm and the worm gear have their force-transmitting contactsurfaces shaped to provide low-noise operation. The tooth bases are eachconcave in shape and the tooth tips are convex in shape. An involutemiddle tooth part is provided in each case between these concave andconvex portions. However, the use of the involute middle tooth partleads to the situation that only a point contact is provided between themeshing teeth in the involute region. As a result, this known worm gearmechanism does not have adequate bearing capacity for high loadsthroughout their meshing region.

[0012] Therefore, there is a need for a power-assisted steering systemthat includes gears with improved coupling.

SUMMARY OF THE INVENTION

[0013] A power-assisted steering system includes a gear mechanism with agear and a mating gear that engage with each other via teeth. The teethhave effective profiles that are matched to each other such that thecontact is relatively linear over the height of the teeth. That is, theface shape is adapted over the height of a tooth to the face shape ofthe tooth meshing with this tooth in such a way that the curvatures ofthe face shapes over the height of the teeth are selected such that to aconcavity is assigned a corresponding convexity on the other tooth, andvice versa. In relation to point contacts as common in the prior art, alinear contact an advantage that the load to be transmitted by one gearto the other gear is distributed over a larger area, so that the loadper unit area of the teeth is decreased.

[0014] In this way, both the wear of the gears and the danger ofoverloading are reduced. Overall, the load-bearing capacity of the gearsis thus increased. The high load-bearing and loading capacity of themechanism is achieved through the practice of providing no involuteregion at all for engagement of the teeth.

[0015] A linear contact over the height of the teeth can be achieved forexample by assigning to a concave region of one tooth a convex region ofthe tooth in engagement with this tooth, the concave region and theconvex region having the same curvature. The tooth base concave in shapeand the tooth tip is made convex in shape, the convex region making atransition to the concave region without the interposition of a furtherregion such as, for example, an involute region.

[0016] The worm gear may be made of a material with a lower strengththan the material of the worm. For example, the worm may be made ofsteel and the worm gear of plastic. The use of a worm gear made ofplastic brings about advantages that relate to fabrication. Plasticgears can be made by the economical injection molding process withoutthe need for subsequent machining.

[0017] The tooth thicknesses of the wheels can be optimized because ofthe reduction of areal load per tooth. Especially good optimizationresults from utilizing the material properties of the material pairings.The tooth thicknesses of the worm gear is preferably greater than thetooth thicknesses of the worm. A reduction in the tooth thicknesses inturn yields cost advantages because material can be saved in this way.

[0018] The worm gear may be cylindrical in shape. In contrast togloboidal worm gears, a cylindrical worm gear has no undercuts. Thisform of worm gears favors the making of the worm gears by injectionmolding, which in turn has a positive impact on costs. In addition, theassembly of worm gear mechanisms with cylindrical worm gears is simplerbecause the worm gear can also be mounted in the axial direction. Noadditional space is required for axial insertion. The danger of damageto the worm gear is greatly reduced with axial insertion as compared toradial insertion. Further, exact axial positioning of the worm gearrelative to the worm is not required, so that the cost of assembly isfurther reduced. What is more, axis angles different from 90° can be setwith a cylindrical worm gear without any change in the load-bearingcapacity of the gears, because the size of the contact area remainsconstant.

[0019] If the worm is made globoidal in shape, the contact over thewidth of the teeth is enlarged. A larger contact additionally reducesthe load per unit area, so that the load-bearing capacity of the gearsis further increased.

[0020] Although the invention is intended and suitable principally foruse in power-assisted steerings of motor vehicles, the invention is notlimited hereto. Instead, the gear mechanism the invention can also beused in window-opening drives, seat adjustments, mass compensationmechanisms, or other adjustment drives.

[0021] These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of preferred embodiments thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

[0022]FIG. 1 is a pictorial illustration of an electromechanicalpower-assisted steering system;

[0023]FIG. 2 is a section parallel to the worm gear axis through thealready mentioned worm gear mechanism with a cylindrical worm gear and aworm with involute tooth pattern engaging with the worm gear;

[0024]FIG. 3 is a section parallel to the worm gear axis through theworm gear mechanism, also already mentioned, with a globoidal worm gearand a worm with involute tooth pattern engaging with the worm gear;

[0025]FIG. 4 is a section parallel to the worm gear axis through a wormgear mechanism according to the invention with a worm gear and a wormengaging with the worm gear;

[0026] FIGS. 5A-5B depict the tooth geometry of the worm gear mechanismof FIG. 4;

[0027]FIG. 6 is a section perpendicular to the worm gear axis throughthe worm gear mechanism of FIG. 4; and

[0028]FIG. 7 is a section perpendicular to the worm gear axis through aworm gear mechanism with a globoidal worm.

DETAILED DESCRIPTION OF THE INVENTION

[0029]FIG. 4 illustrates a worm gear mechanism 400 in a section parallelto the rotation axis A of worm gear 402. The worm gear 402 iscylindrical and has teeth 404 on its outer rim. A worm 406 rotates aboutan axis B that points perpendicularly to the drawing plane. The worm 406is also cylindrical and has teeth 408 on its periphery. The teeth 404 ofthe worm gear 2 engage with the teeth 408 of worm 406. The contact ofthe teeth 404 of the worm gear 402 with the teeth 408 of the worm 406 isidentified as region 410. The contact region 410 extends substantiallyalong height h of the teeth 404, 408 and is linear.

[0030] FIGS. 5A-5B illustrate the tooth geometry of the worm and wormgear. FIG. 5A shows the teeth 408 of the worm 406 pointing downward andthe teeth 404 of the worm gear 402 pointing upward. The teeth 404, 408engage with one another in this depiction as well. The teeth 404, 408each have a height h₄, h₅, respectively. Height h₄ extends from a base502 of tooth 404 to its tip 504. Height h₅ extends from a base 506 oftooth 408 to its tip 508. The width of the teeth 404, 408 varies overthe height h₄, h₅, respectively, and depends on the shape of faces 510,512 of teeth 404, 408. Encircled region 514 represents the regions inwhich teeth 404, 408 make contact when they engage with each other.

[0031]FIG. 5B is an enlarged detail view of the region in FIG. 5A inwhich a region near the tooth head of a worm tooth 408 makes contactwith a region near the tooth base of a worm gear tooth 404. The surfacealong which the teeth 404, 408 contact has a height h₁. Tooth face 514of worm gear tooth 404 is concave in the region of contact. The regionof worm gear 408 engaging with the tooth face 514 is made convex inshape. The two regions have a curvature that is similar or the same atleast piecewise, so that they come into contact in a linear region. Theeffective profiles of tooth faces 510, 512 of worm tooth 408 and wormgear tooth 404 are adapted to each other throughout their height h₄, h₅,(FIG. 5A) so linear contact occurs throughout height h₄, h₅. In theexemplary embodiment depicted, the faces 510, 512 of teeth 404, 408 ofthe worm gear 402 and worm 406 each have a concave profile in theirregion near the tooth base and a convex profile in their region near thetooth tip. With respect to their curvatures, the profiles are adapted toeach other such that in each case regions with curvatures that aresimilar or equal, at least piecewise, come into contact when the teethmesh.

[0032] Linear contact of the teeth has the advantage that the load to betransmitted from one gear to the other gear is distributed over an area,so that the load per unit area decreases. In this way, both the wear ofthe gears 402, 406 and also the danger of overloading are reduced. Theload-bearing capacity of the gears 402, 406 is thus enhanced.

[0033] In one embodiment, the teeth 408 of the worm 406 are thinner thanthe teeth 404 of the worm gear 402. The worm 406 is made of a materialwith a greater strength, such as steel for example, while the worm gear402 is made of a material with a lower strength, such as plastic forexample. The lower load per unit area resulting from the fashioning ofeffective profiles 510, 512 has two advantages in this respect. Thefirst is a substantial reduction in the wear of worm gear, which is madeof the material with lower strength. The second is that the thickness ofthe teeth 404, 408 can be optimized in view of the material pairing, sothat the teeth need not be made thicker than necessary, with resultingmaterial and hence cost savings.

[0034]FIG. 6 shows the previously described worm gear mechanismaccording to an aspect of the invention with the cylindrical worm gear402 and cylindrical worm 406 in a section perpendicular to rotation axisA of the worm gear 402. Rotation axis B of the worm 406 runs parallel tothe drawing plane. The teeth 404, 408 of worm 2 and worm gear 1 againengage with one another. Reference character 13 identifies the region ofengagement of mutually engaging teeth 404, 408. Because of the shape ofthe worm gear 402 and the worm 406, the region of engagement 13 has itsgreatest extent in the direction of the height of teeth at the height ofa central axis C of the worm gear and the worm. This extent decreaseswith increasing distance from central axis C.

[0035] In the case of the use of a globoidal worm 700 (i.e., a worm madeso that region of engagement 13 has the same extent in the direction ofthe height of teeth 702, 704 throughout its length) region of engagement13 can be made much larger as shown in FIG. 7, so that contact area isincreased over the width of the teeth 702, 704. The enlargement of thecontact area has advantages with respect to the load-bearing capacity ofgears, so that the advantages of the invention are increased by the useof the globoidal worm 700. In the exemplary embodiment of FIG. 6, thecylindrical worm gear 402 is still used, which has the advantagedescribed in connection with the fabrication and assembly of gearmechanisms with a cylindrical worm gear.

[0036] Although the present invention has been shown and described withrespect to several preferred embodiments thereof, various changes,omissions and additions to the form and detail thereof, may be madetherein, without departing from the spirit and scope of the invention.

What is claimed is:
 1. A power-assisted steering system having a gearmechanism, coupled to a steering column, with a gear and a mating gear,each having teeth via which they engage with each other, movement andpower being transmitted from one gear to the other gear via effectiveprofiles of their tooth faces, wherein the effective profiles of thetooth faces (11, 12) of gear (1) and mating gear (2) are made such thata linear contact (3) over the height (h₄, h₅) of the teeth comes aboutwhen the teeth (4, 5) engage with each other.
 2. The power-assistedsteering system of claim 1, wherein a convex region with preferablypiecewise at least approximately equal curvature of the tooth face (12)of the mating gear (2) is assigned to a concave region of the tooth face(11) of the gear (1).
 3. The power-assisted steering system of claim 1,wherein a concave region with preferably piecewise at leastapproximately equal curvature of the tooth face (12) of the mating gear(2) is assigned to a convex region of the tooth face (11) of the gear(1).
 4. The power-assisted steering system of claim 3, wherein theconcave region is disposed in a region adjoining a tooth base (6, 8) andthe convex region is disposed in a region adjoining a tooth tip (7, 9).5. The power-assisted steering system of claim 4, wherein each toothface (11, 12) has a concave region and a convex region.
 6. Thepower-assisted steering system of claim 5, wherein the gear mechanismcomprises a worm gear mechanism that includes a worm gear (1) and a worm(2).
 7. The power-assisted steering system of claim 6, wherein the wormgear (1) is made of a material with lower strength than the material ofthe worm (2).
 8. The power-assisted steering system of claim 7, whereinthe tooth thicknesses of worm gear (1) and worm (2) are adapted to thematerial properties of the material pairing of the gears.
 9. Thepower-assisted steering system of claim 8, wherein the tooth thicknessof the teeth (4) of the worm gear (1) is greater than that of the teeth(5) of the worm (2).
 10. The power-assisted steering system of claim 9,wherein the worm gear (1) is made cylindrical in shape.
 11. Thepower-assisted steering system of claim 10, wherein the worm (2) is madegloboidal in shape.
 12. The power-assisted steering system of claim 1,wherein the tooth geometry of the teeth is formed without involutes.